1. Field of the Invention
The present invention relates to a variable-magnification optical system, an imaging optical device, and a digital appliance. More particularly, the present invention relates to, for example, a compact variable-magnification optical system suitable for use in an interchangeable-lens digital camera that takes in an image of a subject with an image sensing device, an imaging optical device that outputs an image of a subject taken in with such a variable-magnification optical system and an image sensing device in the form of an electric signal, and a digital appliance having an image input function, such as a digital camera, that is provided with such an imaging optical device.
2. Description of Related Art
In recent years, mirrorless interchangeable-lens digital cameras, which have no swing-up mirror as are provided in single-lens reflex cameras, have found acceptance among users and gained an increasingly large market. Some mirrorless interchangeable-lens digital cameras cannot adopt phase-difference AF (automatic focusing), which is the mainstream in conventional single-lens reflex cameras; those cameras have to adopt so-called contrast AF, whereby focusing is achieved by scanning for the highest-contrast position with a focusing lens group.
Here, the weight of the focusing group matters. With phase-difference AF, the amount of movement of the focusing group needed to achieve focusing can be calculated based on information from an AF sensor, and thus the focusing group can be moved according to the calculated amount. By contrast, with contrast AF, only the contrast values at given moments can be obtained from an AF sensor; thus, while the focusing group is moved, and meanwhile how the contrast value varies from one moment to another is read, the highest-contract position is searched for to achieve focusing. Thus, the amount of movement of the focusing group needed to achieve focusing is fax larger with contrast AF than with phase-difference AF.
From the above perspective, in a variable-magnification optical system designed to be compatible with contrast AF, a reduced weight of the focusing group is a great advantage (for example, see Patent Documents 1 and 2 identified below). In conventional variable-magnification optical systems for use as interchangeable lenses for single-lens reflex cameras, for example, a positive-negative-positive-positive zoom type as disclosed in Patent Document 3 and a positive-negative-positive-negative-positive zoom type as disclosed in Patent Document 4 are the mainstream. However, from the above perspective, the focusing groups in the lens systems disclosed in Patent Documents 3 and 4 are not satisfactorily light. Thus, new optical solutions are being sought.    Patent Document 1: Japanese Patent Application Publication No. 2012-14005    Patent Document 2: Japanese Patent Application Publication No. 2012-225987    Patent Document 3: Japanese Patent Application Publication No. 2009-271471    Patent Document 4: Japanese Patent Application Publication No. 2011-221422
In the zoom lens system disclosed in Patent Document 1 mentioned above, the second lens group is used as a focusing group, and is not satisfactorily light as a focusing group. In the zoom lens system disclosed in Patent Document 2, the fourth lens group which is composed of a single lens element having a negative optical power is used as a focusing group, and is significantly light as a focusing group. Here, the second lens group in an ordinary positive-negative-positive-positive type zoom lens system is divided into a positive, a negative, and a positive sub groups to set apart the focusing group. In a zoom lens system that includes, from the object side, a positive, a negative, a positive, . . . lens group, magnification variation action ascribable to variation in the distances between the first and second lens groups and between the second and third lens groups accounts for the large part of the magnification variation action of the entire optical system. However, in the construction disclosed in Patent Document 2, as a result of the second lens group being divided, the second to fifth lens groups produce a low zoom ratio, and thus the sixth lens group needs to produce the large part of the desired zoom ratio. Obtaining the desired zoom ratio, therefore, requires an increased total optical length at the telephoto end.
In the lens systems disclosed in Patent Documents 3 and 4, a reduction in the weight of the focusing group may be achieved by performing focusing with a lens group located to the image side of the third lens group which has a small lens diameter. However, in the lens system disclosed in Patent Document 3, the fourth lens group serves for vibration correction, and therefore, arranging a focusing mechanism close to the fourth lens group results in a drive mechanism for focusing and a drive mechanism for vibration correction being arranged close together, which can be disadvantageous from the viewpoint of space efficiency
Conventionally, a vibration correction function is incorporated exclusively in comparatively telephoto-oriented lens systems. However, as zoom lens systems are designed for increasingly high magnifications, more and more zoom lens systems now incorporate a vibration correction function. In this trend, in recent years, even low-magnification zoom lens systems have come to incorporate a vibration correction function. There are also available imaging devices that achieve vibration correction by moving an image sensor on a plane perpendicular to the optical axis. Thus, a vibration correction function is acquiring the status of almost a standard function.